There is no way to customize this. Those entering date information will still need to use the "/" or select from the calendar when trying to enter a specific date. However, there is a shortcut to reduce the number of keystrokes. If you know how many days in the future or past you want to use, you can enter "+" or "-" and the number of days to fill in the field. For example, if the invoice day is exactly 1 week prior to today just highlight the current date and enter "-7" and the full date will fill in. If you want to enter today's date just enter "+0". This works for all date fields not just the invoice date.